This invention relates to a capacitor can housing as well as a method for the production of a capacitor can housing and apparatus for the execution of this process.
The term "capacitor can housing" or herebelow also the shorter term "can housing" includes not only regularly cylindrical capacitor or condensor can housings that are closed on one end, but also can housings open at both ends as well as housings with other than round cross sections.
The provision of a predetermined burst spot on the can housing which is designed to open the can housing by tearing open when the capacitor is overloaded and critical overpressure results within the closed capacitor housing is a safety precaution and thereby prevents the capacitor from bursting explosively or that its closing cover be thrown like a projectile out of its anchoring in the closed can housing.
The predetermined burst spot, resulting from a weakening of the wall, must be disposed in such a manner that the weakest remaining wall thickness still suffices to guarantee absolute imperviousness and resistance to permeation of the can housing up to the required critical pressure threshold.
The preparation of a groove-like weakening of the wall causes, due to the stamping process used, a displacement of material which is raised like a wall, a protrusion or a burr at both sides of the groove produced. Protrusion heights of approximately 0.1 to 0.3 mm at a diameter of the can housing of 6 to 8 cm causes difficulties, particularly in production lines for capacitors. An added step of finishing for cutting-off or grinding-off such protrusions increases unproportionally the price of mass-produced capacitors.
In order to prevent a second step of manufacturing, it was possible to press the capacitor can housing upon a profile-complementary, almost accurately fitting holding mandrel and to form from the outside a wall weakening by means of a stamp device. In that case, the region of the holding mandrel, situated opposite to the stamp device, may be formed as a flat surface region while in all other regions the inner wall of the can housing must abut the holding mandrel exactly and form fitting.
While pressing in the stamp device at first, a bending deformation of the can wall results. This bending deformation is stopped by abutting the inner wall of the can housing to the flattened surface of the holding mandrel. When the stamp edge sinks in further, stamping of the groove results, in other words, the stamping results while the material of the can wall flows. Accordingly, the stamping protrusion or burrs at both sides of the stamped groove are inevitably raised. Due to the deformation of the can wall before stamping, the protrusions or burrs will not protrude over the outer circumferential circle of the can housing but will lie within this boundary. The flattening of the holding mandrel will be chosen at such a measure that the highest elevations of the protrusion or burr lie closely within the highest or upon the outer circumferential circle of the can wall. At a mean burr height of about 0.1 mm, the planar flattening of the holding mandrel will also amount to 0.1 mm or not much above that. For the manufacture of cylindrical can housings out of aluminum at diameters up to about 10 cm, the surface of the planar flattening of the otherwise cylindrical holding mandrel lies preferably 0.05 to 0.5 mm below the vertex of the imaginary superficies of the holding mandrel.
In this example, the planar flattening of the holding mandrel extends its longitudinal extension parallel to the longitudinal axis of the holding mandrel. The longitudinal axis of the flattening, the stamping device and the longitudinal axis of the holding mandrel are preferentially aligned parallel to each other. Thereby the groove-like weakening of the wall runs parallel to the longitudinal axis of the can housing. The flattened surface area in this embodiment may be as well formed along the whole axial length of the holding mandrel as also only along the length of the stamping area.
In the aforedescribed example, protrusions form on the one hand, at the weakening of the wall and, on the other hand, flow of the material in the region of the wall weakening causes deformation stresses and deformation hardening so that the predetermined bursting spot is reduced to a very low wall thickness, for instance to the region between 0.06 and 0.08 mm. Another disadvantageous aspect is that the effect of pressure results in the outwardly arched regions of the area deformed by bending and not at the desired predetermined bursting spot.
Thus an object of the present invention is to provide a capacitor can housing having a greater remaining wall thickness at the predetermined burst spot while considerably avoiding outer bulges of material and deformation hardening in the area of the predetermined burst spot. A process for the fabrication and an apparatus for the execution of this process is also disclosed.
For the prevention of outer material bulges and a large flow of material while fabricating, which leads to work hardening in this area, the predetermined burst spot is provided, according to the present invention, at a deep drawing area of the capacitor can housing. The can wall, deep drawn in the area provided for the predetermined burst spot, is furnished with at least one axially running groove-like indentation. The weakening of the strength of the can-wall caused by deep drawing is slightly equalized by the production of the groove-like indentation. That process prevents the disadvantageous hardening processes caused mainly by cold flow of the material. This makes it possible to plan the remaining thickness of the wall of the predetermined burst spot thicker than heretofore possible. Preferably the remaining thickness of the wall of about 0.12 to 0.13 mm is obtained while in capacitor can housing manufactured according to previous processes, a remaining wall thickness of about 0.06 to 0.08 mm was needed for a triggering or bursting pressure of 14.+-.2 bar. This increase of the remaining thickness of the wall at the predetermined bursting spot by almost a factor of 2 improves thereby the permeability strength of the can housing and offers on the other hand, a greater safety in regard to the technique of manufacturing, particularly in the automatic steps of processing in mass production.
Compared to the aforedescribed capacitor can housing, the can housing according to the invention has a relatively narrowly defined, precise area for the weakening of the wall and the predetermined burst spot, resulting from the deep drawing in a relatively narrow groove. By substantially preventing work hardening and hardening due to deformation in the deep drawn can housing, the predetermined burst spot is really also the area of minimum wall thickness so that bursting by excess pressure occurs for safety reasons only at the planned spot, in other words at the predetermined burst spot.
This kind of a predetermined burst spot may be realized advantageously in can housings of capacitors having for instance outer diameters of 12, 25 or 35 to 40 mm, and corresponding wall thicknesses of 0.35, 0.40 or 0.45 mm, respectively. The convex shaping of the outer wall of the predetermined burst spot and the approximately straight form of the inner wall have the advantage that these can housings may be pushed on or pulled off, respectively, a holding mandrel under conditions of mass production. It is additively supportive for easier handling in manufacturing processes to allow a play of about 1 mm between the diameter of the holding mandrel at the outer diameter of about 12 mm for the can housing. The deep drawing of about 0.3 mm relative to the normal inner circumferential radius of the can housing is in that case without deleterious influence upon the subsequent introduction of capacitor windings into the interior of the can housing.
The bilateral forming of an indentation in the can wall, in other words as well inside as outside, succeeds in making the predetermined burst spot lying approximately in the neutral central plane of the can wall even after the deep drawing process. The shaping of the can housing with two groove-like indentations of approximately equal width is particularly conducive for this embodiment. Here the bottom of the inner indentation lies advantageously within the inner radius of the circumferential wall of the can in which case a radius, smaller by about 0.15 mm, is particularly appropriate in this embodiment.
In another preferred embodiment, the inner indentation protrudes over the circumferential circle of the inner wall of the can housing radially outwardly, in which case it is desirable to use a radius larger by about 0.1 mm than the inner radius. In this embodiment, an indentation wider than the inner indentation is provided in the outer side of the can wall. The breadth of the deep drawn region of the can housing may advantageously amount to 1.5 mm at the inner circumference while the inner bulge protrudes due to deep drawing about 0.3 mm into the interior of the can housing.
The forming of the can housing in such a manner provides that in its final stage, the center of the wall thickness of the predetermined burst spot lies after deep drawing approximately in the center of the neutral center plane of the can wall and prevents or equalizes influences arising from a greater material hardness in the edge area of the can housing. Thus, in this embodiment, it is possible to determine relatively exactly the remaining wall strength or thickness of the predetermined burst spot dependent on the predetermined bursting pressure and also to put it in practice by following tried manufacturing methods.
The process of manufacturing the aforementioned capacitor can housing includes pushing the can housing onto a holding mandrel and then deep drawing the housing wall in the area of the predetermined burst spot to form a groove on at least one housing wall side in the deep drawn area. The bursting of a capacitor can housing prepared by this method within the remaining wall strength occurs approximately in such a manner that a tearing apart of the predetermined burst spot by a corresponding critical tensile stress occurs at the threshold pressure. Manufacture according to tried methods of the can housing by using a holding mandrel is facilitated by the fact that the generally deep drawn can housing has in the axial walls draw flutes in the region of about 10 to 15% of the wall strength.
A device for execution of the process includes a substantially cylindrical holding mandrel serving as the inner die against an outer die, acting from the outside. The holding mandrel is furnished with a groove which may reach as well along the whole length as also only along a part of the axial length of the holding mandrel, and which now corresponds to the position and the length of the wall weakening provided for the predetermined burst spot. The groove is preferably U-shaped, in which case a groove-breadth of about 1.5 mm and a groove height from groove bottom to groove rim of about 0.3 mm have been successfully used. Preferably the groove bottom is formed as an even space from which almost vertical groove walls rise up to circumferential wall of the holding mandrel. Another groove, though, may be chosen, for instance a groove provided with chamfers towards the axis of the holding mandrel.
The outer die of the device is shaped so that it has a breadth smaller than the breadth of the groove. It is possible to use, for instance, a relation of 1.5 mm breadth of the groove and 0.5 mm breadth of the outer die on the narrow side. In an axial top view of the outer die, its longitudinal profile is preferably shaped so that an obliquely running chamfer to the narrow sides is provided, the chamfer originating in a center area running approximately parallel to the axis of the holding mandrel. The central area has a length of approximately 4 mm and the areas of the chamfer amount to approximately 0.1 mm. The angle between the extension of the median area, for instance, and the chamfer may amount to 30.degree. to 45.degree.. The transition into the narrow side is provided by a chamfer having a radius of about 0.05 mm. The chamfer has the advantage that punctures due to deep drawing are avoided, thereby diminishing the safety of the predetermined burst stop. The head of the outer die is approximately trapezoidally-shaped in its narrow side profile when it is formed as a groove die.
The groove has preferably a preferentially centrally disposed groove die, which may be provided as well over the whole length of the groove as only in the area of the predestined burst spot to be formed. While the outer die has bent rim areas which are in their final effect responsible for the convex configuration of the outer wall of the predetermined burst spot, the groove die, acting from inside, is shaped in a straight line over its whole length and verges with a very small chamfer into the narrow sides.
Although the device is equipped with a groove die, protruding radially above the circumferential circle of the holding mandrel, it is possible to choose preferably a groove die with a wedge point, the die being shorter and lying below the circumferential circle where, in the last mentioned embodiment, the forming of the outer die as a groove die results in additional advantages. The inner groove die has advantageously a height of about 0.1 mm, measured from the groove bottom, at a whole groove height of 0.3 mm. A height slightly below the groove height may also be chosen.
The use of two oppositely acting groove dies and a corresponding construction of the groove results in extremely advantageous remaining strengths and thicknesses of the predetermined burst spot. The predetermined burst spot lies, when using this method of fabrication, approximately in the region of the neutral center space of the can wall so that an easily adjusted remaining wall strength may be obtained, substantially without deformation hardening and without fluctuations of wall strengths caused by deep drawing flutes.
Other features which are considered characteristic of the invention are set forth in the appended claims.
Although the invention is illustrated and described in relationship to specific embodiments, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope or range of equivalents of the claims.
The construction and operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.